This invention relates generally to radiotelephones and, in particular, to radiotelephones or mobile stations capable of operation with a digital wireless telecommunications network.
One modern wireless telecommunications system is known as the Global System for Mobile Communication (GSM) Evolution (EDGE), which includes Enhanced Circuit Switched Data (ECSD) and Enhanced Global Packet Radio Service (EGPRS). The teachings of this invention are particularly relevant to the ECSD aspect, as well as to other types of TDMA systems, such as the IS-136 system in the United States, having circuit switched services.
During the EDGE standardization process some mobile equipment or mobile station manufacturers expressed a concern that they were not able to immediately support 8-PSK modulation in their transmitters. The 8-PSK modulation was selected for GSM, in addition to the Gaussian Minimum Shift Keying (GMSK) type of modulation that was already in use. Some manufacturers also expressed an interest in providing low cost mobile equipment that would not support 8-PSK transmissions.
As may be appreciated, these limitations make it very difficult to support high bit rate circuit switched services with 8-PSK modulation, without changing the manner in which these services are defined in the GSM specification.
Currently, the GSM specification defines circuit switched services as xe2x80x9csymmetricxe2x80x9d services, that is, using the same modulation/channel coding in both the uplink direction (mobile station (MS) to base transceiver station (BTS)) and in the downlink direction (BTS to MS). Furthermore, there were no proposals as to how to support ECSD services for those xe2x80x9csimplexe2x80x9d mobile stations that were unable to transmit with 8-PSK modulation in the uplink direction.
One problem that results from this situation, in the case of circuit switched GSM or some other xe2x80x9csymmetricalxe2x80x9d systems, is in enabling the use of different types of modulation on the same radio interface, wherein the xe2x80x9csimplexe2x80x9d mobile stations cannot make full use of the system capabilities without having a complex transmitter that supports all system modulation schemes. This is due largely to the current symmetrical nature of the radio interface, where the same modulation/channel coding is used for both MS reception (downlink) and MS transmission (uplink).
Another problem that results in the case of the above-mentioned system(s) is that if a downlink biased data transfer is required by the MS, for example the transfer of a large file on the downlink, with a xe2x80x9cmore complexxe2x80x9d MS that supports several of the system modulation schemes, the MS must, according to current GSM specifications, use the downlink modulation/channel coding also for uplink transmission. However, this type of transmission is not optimal from the MS power consumption and/or uplink robustness point of view. That is, and if one assumes a sufficient amount of BTS transmitter power, the ECSD link can be maintained for a longer distance if GMSK modulation is used in the uplink, as opposed 8-PSK modulation. This is particularly true when the MS operates with limited transmitter power classes. It should be noted in this regard that even if the MS output power is equal for 8-PSK and GMSK, the link budget is inferior for the 8-PSK modulation case.
It is a first object and advantage of this invention to provide a technique for accommodating mobile stations with varying modulation capabilities in a wireless telecommunications system.
It is a second object and advantage of this invention to provide wireless communications system with an ability to selectively provide symmetrical or asymmetrical data services in the downlink or the uplink, and to further enable different types of modulation and bit rates to be used in the downlink and the uplink.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
The teachings of this invention provide a data transmission method and system (e.g., GSM EDGE) for circuit switched and other services that is employed, preferably, in TDMA-based systems, and that supports the use of one or several types of modulation (e.g., 8-PSK modulation), in addition to a xe2x80x9cnormalxe2x80x9d modulation (e.g., GMSK modulation) on the radio interface. An additional modulation/channel coding (e.g., one based on 8-PSK) may be used in the downlink transmission, while the normal modulation/channel coding (e.g., one based on GMSK) may be used in the uplink transmissions, under a variety of different conditions.
A first condition arises where the radio conditions allow the use of the additional modulation in both directions, but the mobile station transmitter does not support the additional modulation. For example, the mobile station may be capable of receiving 8-PSK downlink transmissions, but is only capable of GMSK modulated uplink transmissions.
A second condition arises where the radio conditions allow the use of the additional modulation in both directions, and the mobile station supports the additional modulation, but the user requests a downlink-biased (or an uplink-biased) data transmission service.
A third condition arises where the mobile station supports the use of the additional modulation in both directions, but the uplink (or the downlink) radio conditions do not permit the use of the additional modulation (e.g., because of link budget limitations.)
Under all of the foregoing conditions, the same Layer-3 (L3) protocol format is used in both directions for transferring the data frames (e.g., 14.5 kbps Radio Link Protocol (RLP)).
Under the second condition, the mobile station automatically uses the normal modulation in the uplink according to the user""s request, and the base station uses a blind detection of the uplink bursts to identify the type of modulation used for the uplink transmissions.
Also under the second condition, the mobile station indicates during call setup signaling that the downlink biased service is requested by the user, and the network establishes the connection accordingly.
Under the third condition, if the link budget is not sufficient for 8-PSK modulation (or some other type of additional modulation), the network may command a power control level that is not supported by the mobile station for the additional modulation. In this case the mobile station will, preferably, use a nearest supported output power control level.
The teachings of this invention enable the use of different data rates for the uplink and the downlink, thus providing improved radio resource utilization, a simpler ECSD mobile station implementation and, assuming a reduced power consumption for the ECSD mobile station, an extended range of the cell for ECSD services. Further in this regard, and as was indicated above, even if the mobile station output power is equal for both the 8-PSK and GMSK modulation cases, the link budget is generally inferior for 8-PSK.
Also, a lower data rate may be desired on the uplink in order to reduce the power consumption of the mobile station, and/or in the downlink direction to avoid unnecessary interference. This is especially true in the case that one direction or the other has less data to transmit, and the high speed data transmission mode is not required (even if supported).